We are studying the effect of surface mutations on the thermodynamic stability of yeast iso-1-cytochrome c. Lysine 73, a highly solvent exposed surface residue, has been replaced with aromatic and aliphatic amino acids using site directed mutagenesis. Previous experiments with guanidine hydrochloride denaturations monitored with circular dichroism revealed smaller m values for these mutants as compared to the wild type protein. Thermal denaturation studies as a function of pH were carried on the wild type and trp73, ile73, val73 variants. Further experiments with differential scanning calorimetry so as to probe the possibility of intermediates causing smaller m values for these variants may yield useful information for interpretation of the data.